1. Field of the Invention
This invention relates to balun circuits for coupling between balanced and unbalanced lines or devices in an electronic system. More particularly, this invention relates to a miniaturized wideband multi-layer balun circuit for use in microwave and RF applications such as mobile communication devices.
2. Description of Related Art
Typically, a balun is used to couple a differential (balanced) circuit, such as a balanced amplifier, to a single-ended (unbalanced) circuit, such as an antenna. The following references provide background information relating to baluns and are incorporated by reference herein in their entireties:
[1] U.S. Pat. No. 4,994,755 to Titus et al., entitled "Active Balun," Feb. 19, 1991; PA1 [2] U.S. Pat. No. 5,039,891 to Wen et al., entitled "Planar Broadband FET Balun," Aug. 13, 1991; PA1 [3] U.S. Pat. No. 5,574,411 to Apel et al., entitled "Lumped Parameter Balun," Nov. 12, 1996; PA1 [4] S. A. Maas, "Microwave Mixers", Artech House, pp 244-255, 1933; PA1 [5] U.S. Pat. No. 5,455,545 to Garcia, entitled "Compact Low-loss Microwave Balun," Oct. 3, 1995; PA1 [6] U.S. Pat. No. 4,725,792 to Lampe, Jr., entitled "Wideband Balun Realized By Equal-Power Divider and Short Circuit Stubs," Feb. 16, 1988; PA1 [7] U.S. Pat. No. 4,460,877 to Sterns, entitled "Broad-Band Printed-Circuit Balun Employing Coupled Strip All Pass Filter," Jul. 17, 1984; PA1 [8] U.S. Pat. No. 5,497,137 to Fujiki, entitled "Chip Type Transformer," Mar. 5, 1994; PA1 [9] U.S. Pat. No. 5,025,232 to Pavio, entitled "Monolithic Multilayer Planar Transmission Line," Jan. 18, 1991; PA1 [10] U.S. Pat. No. 4,847,626 to Kahler et al., entitled "Microstrip Balunz-Antenna," Jul. 11, 1989; PA1 [11] U.S. Pat. No. 4,755,775 to Marczewski et al., entitled "Microwave Balun for Mixers and Modulators," Jul. 5, 1988; PA1 [12] U.S. Pat. No. 5,172,082 to Livingston, et al., entitled "Multi-octave Bandwidth Balun," Dec. 15, 1992; PA1 [13] U.S. Pat. No. 5,296,823 to Dietrich, entitled "Wideband Transmission Line Balun," Mar. 22, 1994; PA1 [14] U.S. Pat. No. 5,534,830 to Ralph, entitled "Thick Film Balanced Line Structure, and Microwave Baluns, Resonators, Mixers, Splitters, and Filters constructed Therefrom," Jul. 9, 1996; and PA1 [15] U.S. Pat. No. 5,697,088 to Gu, entitled "Balun Transformer," Dec. 9, 1997.
The term "balun" is a contraction of balanced to unbalanced. A balun is a RF balancing network or electric circuit for coupling an unbalanced line or device and a balanced line or device for the purpose of transforming from balanced to unbalanced or from unbalanced to balanced operation, with minimum transmission losses. A balun can be used with an unbalanced input and a pair of balanced outputs or, in the reverse situation, a pair of balanced sources and an unbalanced load. Baluns can be used to interface an unbalanced input with a balanced circuit by dividing the signal received at its unbalanced terminal equally to two balanced terminals and by providing the signal at one balanced terminal with a reference phase and the signal at the other balanced terminal with a phase equal to the reference phase plus or minus 180.degree.. Baluns can be used to interface a balanced or differential input from a balanced port of a balanced circuit providing output signals which are equal in magnitude but 180.degree. out-of-phase and an unbalanced load driven by a single-ended input signal. The balun combines the signals of the balanced input and provides the combined signal at an another port.
The balanced structure is usually needed in devices such as balanced mixers, modulators, attenuators, switches and differential amplifiers, since balanced circuits can provide better circuit-to-circuit isolation, dynamic range, and noise and spurious signal cancellation. A balanced load is defined as a circuit whose behavior is unaffected by reversing the polarity of the power delivered thereto. A balanced load presents the same impedance with respect to ground, at both ends or terminals. A balanced load is required at the end of a balanced structure to ensure that the signals at the balanced port will be equal and opposite in phase. Depending on the implementation, baluns can be divided into two groups: active and passive. Active baluns are described in references [1] and [2] and are constructed by using several transistors (so-called active devices). Although active baluns are very small, they are not generally preferred for the following reasons. First, due to the employment of active devices, noise will be introduced into the system. Also, active devices tend inherently to waste power; this makes them quite disadvantageous in radio telephone systems. Additionally, the low-cost fabrication of active baluns is limited to semiconductor manufacture. Conversely, passive baluns are quite popular. Passive baluns can be categorized into lumped-type baluns, coil-type baluns, and distributed-type baluns.
Lumped-element-type baluns are described in references [3] and [4]. Lumped-element baluns employ discrete components that are electrically connected, such as lumped element capacitors and lumped element inductors. Advantages of lumped-element-type baluns include small size and suitability for low frequency range usage. On the other hand, the performance of lumped-element-type baluns is not good in high frequency ranges (several Ghz), because the lumped elements are very lossy and difficult to control. Also, the operational bandwidth of lumped-element-type baluns is small (&lt;10%, typically).
Coil-type baluns (trifilar transformers) are very popular in applications in the UHF band or lower frequency range. Shortcomings of the trifilar transformer include unacceptable lossiness in the frequency range higher than the UHF band, and barriers to miniaturization beyond a certain size.
There are many kinds of distributed-type baluns. A first type is the 180.degree. hybrid device described in references [4] and [5]. They are constructed by several sections of quarter-wavelength transmission lines and a section of half-wavelength transmission line. The drawbacks of the 180.degree. hybrid device are larger size, difficulty in achieving a high impedance transformation ratio, and limitation to a balanced pair of unbalanced outputs. A second type is the combination of a power divider and a 180.degree. phase shifter as described in references [6] and [7]. Since the 180.degree. phase shift is achieved by a half-wavelength length difference, the size is still too large. A third type is the well-known Marchand-type balun as described in references [8]-[11]. This type of balun has very wide bandwidth (multi-octave). Further, both the phase balance and the amplitude balance are excellent. Moreover, it can be applied not only in a balanced port (load) but also in a balanced pair of unbalanced transmission lines. A fourth type of balun can be classified as a transmission line balun as described in references [12]-[15]. This type of balun uses various manners of connections of coupled transmission lines for implementation and can usually provide satisfactory performance and bandwidths.
In general, low return loss, low insertion loss, and good balanced characteristics are required for balun applications. In addition, bandwidth is another figure of merit. A wideband balun can be used in applications where a wide range of frequencies is present, and alternatively, it can provide a single-device solution to many different narrow frequency band problems. Furthermore, wideband baluns can tolerate more fabrication variation in band-limited applications. However, most of the known wideband balun structures have relatively large sizes, which is sometimes unacceptable in modem wireless applications.